Tissues and organs of animals and humans develop characteristic vascular architecture to meet specific nutritional and physiologic requirements. Each vascular system comprises networks of large and small blood vessels, wherein each vessel has a different flow rate and function. Under various physiological and pathological conditions, new micro-vessels are formed from existing vasculature. This process of new vessel formation, called angiogenesis, occurs through tightly regulated processes under normal physiological situations.
When these tight controls are breached, uncontrolled angiogenesis ensues. This is often accompanied by the development and progression of a variety of inflammatory, neoplastic, ocular and other diseases. In each situation, however, there is a need to measure blood flow through new and existing blood vessels.
Doppler ultrasound is currently used to measure blood flow through relatively large blood vessels (e.g., >0.2-0.5 mm diameter). However, it is not possible to use Doppler ultrasound to measure the flow through small or newly developed angiogenic vessels. This is, in part, due to the weakness of the signal from the blood moving through such tiny vessels. The use of ultrasound contrast agents could alleviate the signal problem; however, the majority of contrast agents used to enhance ultrasound gray-scale (GS) and Doppler (D) images consist of microbubbles. When these agents are injected, the ultrasound images are enhanced over time, but their use to date for measuring the flow in small vessels has been very limited because the microbubbles are unstable at ambient pressure, causing the bubbles to collapse.
Many studies have demonstrated the use of ultrasound contrast agents (UCA) to enhance images and to determine tissue vascularity, perfusion and blood flow. The image-enhancement versus time plots (often referred to as dilution curves, or video-density-time-curves, or time-intensity (TI) plots) follow a skewed bell-shaped curve with characteristics similar to those observed for indicator dilution curves. However, it has been observed that the microbubbles that produce enhancement are destroyed by the pulses of ultrasound used for constructing images (Porter et al., Circulation 92:2391-2395 (1995)).
Gas filled microbubbles that are used in contrast-enhanced imaging are hemodynamically inert and have a rheology similar to that of red blood cells. They are also small and uniform in size, making them less prone to axial streaming or geometric exclusion than has been reported with the use of larger radiolabeled microspheres. Furthermore, their size permits the microbubbles to remain entirely intravascular. Repeated measurements are possible because the microbubbles are rapidly cleared from circulation following administration, but image enhancement is short-lived.
To overcome this problem, techniques related to intermittent imaging have been proposed in which the scanner is turned on/off at a predetermined interval or with EKG gating (Porter et al., 1995). However, while that approach prolongs the time of enhancement, it does not provide quantitative measurements of blood flow or of any other related parameters.
A method based on replenishment of bubbles in scan plane was proposed by Wei and coworkers (Wei et al., Circulation 97:473-83 (1998); Wei et al., J. Am. College of Cardiology 37(4):1135-1140 (2001)). The method is conducted by infusing contrast agent and measuring the replenishment of bubbles as a function of scan interval. The flow velocity and flow rates are derived from the measurement of initial increase in the rate-of-replenishment and the leveling of video intensity value.
Nevertheless, the method of Wei et al. has several limitations. It only works when long infusions of contrast agent are made, and over this entire duration, a steady state of contrast must be achieved. The interval between images acquired is usually large, and can be as high as 30 seconds. During this interval, there is considerable motion, making it difficult to keep the plane fixed during data acquisition. Moreover, the method also requires image matching for background subtraction, which in view of the motion can be cumbersome and difficult to implement. Furthermore, the method fails to take into consideration the attenuation effect of the overlying tissue, or the bubble destruction during the transit of ultrasound through the scan plane.
In the early stages of ultrasound contrast development, methods similar to those used in tracer techniques were proposed, using ultrasound contrast agents to measure blood flow and perfusion (Rovai et al., 1993, In Role of Echo-Contrast in Quantitative Analysis, (Nanda & Schlief, eds), Kluwer Academic Press, Dordrecht, Netherlands, pp. 341-357; Kaul, Am. J. Cardiac Imaging 5(3):200-216 (1991); Sehgal et al., J. Ultrasound Med. 16:471-479 (1997); Sehgal et al., J. Ultrasound Med. 14:741-748 (1995); Sehgal et al., J. Ultrasound Med., 17:751-756 (1998); Arger et al., J. Ultrasound Med., 18:843-851 (1999)). These methods are based on measuring the area under the video-intensity-time curves and the mean transit time following bolus injection of contrast agent.
The problem, however, with the conventional method of image enhancement is that the amount of contrast agent entering an organ not only leaves with the outflow of blood, but it is also removed from circulation due to the destruction of microbubbles. This unique behavior of the ultrasound contrast agent decreases the time taken by the contrast agent to transit or clear through the organ. It also significantly reduces the area under the dilution (time-intensity) curve. Because the blood flow perfusion and the extent of vascularity are related to these measurements, the underestimation of area under the curve and the time of transit lead to erroneous measurements. Therefore, a direct measurement of flow (or any other related parameter) cannot be made from the video-intensity curves unless the effect of bubble destruction is taken into consideration. However, until the present invention, the separation of the two effects, (i.e., (1) removal of the contrast-agent due to flow, versus (2) its removal due to bubble destruction) had not yet been achieved. Thus, there had remained a need for a means and apparatus that would separate the two effects, while at the same time preserving the near real-time capabilities.